This invention concerns an inductive displacement transducer suitable for indicating the displacement of a control rod in an internal combustion engine, for example the control rod of a fuel injection mechanism determining the amount of fuel injected per cycle in an engine cylinder. In particular the invention concerns the kind of transducer that utilizes a semi-differential variable inductance in which inductance variation is produced by mechanical motion of a short-circuiting ring along a magnetic core providing a magnetic path for the inductor. By a semi-differential variable inductance is meant a variable inductance unit which includes also a comparison inductance independently adjustable but kept at a fixed setting in the use of the unit. Semi-differential circuits do not have the degree of balance that full differential circuits have, for example FIGS. 8 and 9 of U.S. Pat. No. 4,132,980 where the inductance of one winding is reduced as that of another winding is increased.
It is known to use a semi-differential inductance unit combining a variable inductor responsive to mechanical displacement and an independent comparison inductor, together with an evaluation circuit by which a signal representative of the displacement is obtained. Apparatus of this type is useful, in the electronic control of a diesel engine, for determining distances of travel and angles, particularly with respect to the control rod of a fuel injection pump. In such an arrangement the short-circuiting ring of the variable inductor is coupled to the control or regulation member of the pump so as to move with it. The evaluation circuit needed by apparatus of this type heretofore known is quite complicated on account of the necessity of taking into account the comparison inductance as well as the variable inductance in continuous measurement. In our copending application Ser. No. 190,490, filed Sept. 25, 1980, we disclosed a much simpler circuit capable of providing, with a semi-differential variable inductor, a signal indicative of the ratio of the variable to the comparison inductance. It is desirable, however, to provide a signal more sensitive to variation in displacement.